I know this has been discussed to death on the internet but fuck it, I'm going in!
If you exclude Candy Crush stuff then I'd say there's less than 50% by quite a chunk but there's also many variables. Generation being one. Lots of girls I knew played games in the late 80s/early 90s. Stuff like Pac-Man, Tetris, Paperboy, New Zealand Story, Rainbow Islands, Double Dragon, Bubble Bobble and so on. Then all of them reached their teens and, for whatever reason, left games behind.
I can tell from the reactions of male gamer friends and their mates that it's rarer to find older gamer ladies. I think it's a lot more common nowadays for girls to continue gaming. It's now a huge business and lost a lot of the 'eww it's uncool and nerdy' reputation. Geeky stuff in general has become massive - Marvel/DC movies, board games surge in popularity. You can now trip over gamer/geek girls all over the internet. It's nothing special or unusual.
Then there's the differences in genre and taste. Only myself and one other female friend enjoy online shooters and multiplayer in general. The others prefer to game alone with retro titles or fantasy RPGs.
Purely anecdotal and I can't be bothered to accurately count but out of roughly 60ish people on my gaming friends list, only about 5 are female. If I were a decade or two younger though then I wonder if there'd be more. Not that it's something I dwell on. I don't care what gender I'm gaming with as long as we're having a laugh.
That's my sperging for the day done.